Pliable sheets of heat sealable materials are commonly used in many industrial, environmental and civil construction applications. For instance, a heat sealable sheet is utilized as a roofing membrane to prevent leakage of water through a roof. Heat sealable sheets are also used as pond and landfill liners to contain hazardous waste and to prevent the waste from seeping into the ground. Porous thermoplastic materials such as woven and nonwoven textiles are used for soil erosion control and for lateral filtration in waste management. Typically, in such applications, large sheets of heat sealable materials are placed on the surface to be covered i.e. pond bottom or roof etc., with their edges overlapping. The overlapping edges are then joined by either sewing them together or seam-welding or tack-welding them to each other along or within the overlap.
If the selected process for joining the sheets is a seam or tack welding process, an artisan first separates the overlapping edges and then heats the opposing surfaces of the separated edges by either directing hot air between the separated edges with a blowing hot air device or by heating the opposing surfaces with a heated wedge or platen. When heated sufficiently, the overlapping edges are pressed together to provide a seam or tack-weld between the overlapping sheets which joins the sheets together when it cools.
The above mentioned sewing and hot air methods are very time-consuming and, therefore, expensive. The aforementioned heated wedge or platen method is much quicker and, thus, less costly than the other methods. However, this method also has a drawback in that movement of the wedge between and along the overlapping edges tends to cause the overlapping edges to laterally separate, i.e. spread away from each other such that their edges move out of parallel alignment with each other. As will be appreciated, this not only makes it difficult to maintain a uniform weld but also increases the time required to complete a weld because to keep the edges aligned, the artisan has to occasionally stop welding and manually realign the overlapping edges.